Foam Uses Expand

If you think free-rise urethane foam is just something you sit on, think again. "It's not always considered an engineering material," laments Andrew Thompson, a polymer chemist and executive vice president of Foamex International's technical products group.

But it should be. The engineers at Foamex have cooked up a variety of new foam formulations that add all kinds of desirable engineering properties to a material often consigned to nothing more than couch cushions and car seats. Among Foamex's new products are patent-pending foams that do a better job than ever at conducting electricity in digital imaging equipment and absorbing energy in computer disc drives. The company's most recent foam developments may also prove useful in automotive, medical, and packaging applications.

Electric foams

To provide an alternative to the elastomer electro-conductive rollers in digital imaging equipment, Foamex has recently come up with high-density free-rise foams that exhibit electrical resistivity values from 106 to 1012 ohms/sq. "We've replicated the functionality of solid elastomers in a free-rise foam," says Chiu Chan, a Ph.D. chemical engineer who runs Foamex's R&D group.

The concept of conductive urethane foams may already ring a bell. Free-rise foams with densities from 2 to 4 lb/ft3 have seen some use in digital imaging applications, but this low density limited their use to less demanding rollers. Of the seven or so rollers found in a typical laser printer, for example, only one of them could typically employ the traditional low-density foams, notes Thompson. What's more, the electrical properties of the low-density foams stemmed from carbon coatings, which can wear off over time.

The company's new ElectrocellTM products take a different approach. Thanks to patent-pending developments in urethane chemistry and processing methods, these foams incorporate metallic salts, carbon, or other conductive additives into the polymer rather than relying on coatings. Building the conductivity into the polymer formulation not only solves the coating wear problem but also ensures that the electrical properties are evenly distributed through the foam, Chan explains.

Foamex engineers have managed to incorporate various conductive additives while simultaneously improving on density and pore size—two key physical properties that govern the foam's firmness and resistance to toner build-up. Densities of the new Electrocell materials can top 10 lb/ft3, versus a maximum of 4 lb/ft3 with Foamex's traditional conductive foams. Pore size has dropped too. The new, finer foam contains between 200 and 300 pores/inch, down from a maximum of 100 pores/inch in the past. "Pore size is at historically unheard of levels," says Thompson.

So why switch from elastomer in the first place? With just one-sixth to one-eighth the density of conductive elastomers, Electrocell promises a cost reduction that boils down to saving material. "We can do the same job using less material," Thompson says.

A quiet spin

Another recent foam development addresses tightening noise and vibration requirements for disc drive mounting materials. Five years ago, Foamex began development of foams that could both absorb sound and dissipate the vibration energy from the spinning hard drive discs. "It turns out the foams that do a good job acoustically don't usually do a good job with mechanical energy," Chan says, explaining that foams optimized to resist deflection under mechanical loads don't usually have the same chemical composition and cell structure as those optimized to absorb sound waves.

To come up with foams that handle both the acoustic and mechanical energy, Foamex engineers sifted through thousands of possible urethane formulations until they found ones that did the trick. Noise reduction proved to be the most difficult property to deliver. When the first generation of these foams appeared five years ago, they managed to hold disc drive noise to 40 dB across a frequency range from 60 to 10,000 Hz, Chan reports. "Then the OEMs dropped the allowable noise," Chan says, which sent Foamex's engineers back to the lab.

Just this year, Foamex came up with new foam formulations that can promise to meet new noise requirements of 35 dB across that same frequency range. And Chan says the foams at some frequencies have been able to keep noise as low as 30 dB. These new foams are currently being put through their paces.

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